Image forming apparatuses and image reading apparatuses using a lens array optical system (imaging optical system) including a small-diameter lens array have been developed recently. For example, image forming apparatuses and image reading apparatuses that include a built-in unit in which a lens array optical system is held in a housing with an array light source (light-emitting diode (LED)) or a line sensor have been known. The use of the lens array optical system can reduce the size and cost of such apparatuses.
A lens array optical system has the problems that an imaging light amount and imaging performance on an image plane (referring to a sensor surface in the case of an image reading apparatus, and a photosensitive surface in the case of an image forming apparatus) tend to drop, and that variations occur in the imaging light amount and the imaging performance. Techniques for solving such problems are discussed in Japanese Patent Application Laid-Open No. 63-274915 and U.S. Patent Application Publication No. 2008/0080057.
Japanese Patent Application Laid-Open No. 63-274915 discusses a lens array optical system in which a plurality of lens optical systems is arranged in one direction (main array direction). The lens array optical system is configured so that the plurality of lens optical systems is arranged in a single row when seen in a direction (sub array direction) perpendicular to the main array direction and to an optical axis direction. Light fluxes incident on the respective plurality of lens optical systems form erect equal-magnification images in a cross section perpendicular to the sub array direction, and inverted equal-magnification images in a cross section perpendicular to the main array direction. According to such a configuration, lens power needed in the sub array direction is lower than with an optical system where erect equal-magnification images are formed in the cross section perpendicular to the main array direction. Favorable imaging performance can thus be obtained even with smaller F values, which can ensure compatibility between the imaging light amount and the imaging performance.
U.S. Patent Application Publication No. 2008/0080057 discusses a lens array optical system in which two lens optical system rows each including lens optical systems arranged in a main array direction are arranged in a sub array direction. The lens optical systems are arranged in a staggered arrangement. More specifically, the optical axes of the respective lens optical systems of the lens optical system rows are separate from one another in the main array direction. According to such a configuration, the number of lens optical systems for a light flux from each light emitting point of an array light source to pass through can be increased to average the imaging light flux at each light emitting point position. As a result, variations in the imaging light amount and the imaging performance can be reduced.
The lens array optical system discussed in Japanese Patent Application Laid-Open No. 63-274915 includes a single row of lens optical systems in the sub array direction. Such a configuration is disadvantageous in terms of reducing variations in the imaging light amount and the imaging performance. The lens array optical system discussed in U.S. Patent Application Publication No. 2008/0080057 is not applicable to a system where inverted images of an object are formed in the cross section perpendicular to the main array direction. Such a configuration is disadvantageous in terms of ensuring comparability between the imaging light amount and the imaging performance.
Suppose that lens optical systems are arranged in two rows in the sub array direction (Z direction) as discussed in U.S. Patent Application Publication No. 2008/0080057. FIG. 36 illustrates cross-sectional views of such configurations perpendicular to the main array direction (ZX cross-sectional views). Each lens optical system includes two lenses juxtaposed in the direction of an optical axis which is illustrated by a dashed-dotted line (X direction). The lenses are represented by ideal lenses (arrows in the diagram). As can be seen from FIG. 36, in the system that forms erect equal-magnification images of an object, the upper and lower lens optical systems form the images in the same position. On the other hand, in the system that forms inverted images of an object, the lens optical systems form the images in separate positions in the sub array direction. The imaging positions do not coincide with each other.
That is, simply combining the techniques discussed in Japanese Patent Application Laid-Open No. 63-274915 and U.S. Patent Application Publication No. 2008/0080057 cannot provide a solution to both the problems that the imaging light amount and the imaging performance tend to drop and that variations occur in the imaging light amount and the imaging performance.